EP-A-1 095 634 (EP 634) discloses all features of the preamble of independent claims 1 and 11. EP 634 discloses a system in which the soft atraumatic distal tip of the system is at the leading end of the inner catheter. The outer sheath of the delivery system has a distal end which stops proximally short of the atraumatic tip.
Stents to be deployed at a stenting site within a human or animal body expand radially in the course of delivery, from a radially compact delivery disposition to a radially larger deployed disposition. In self-expanding stents made of stainless steel, the deformation of the stent is below the elastic limit, the stent until its deployment being radially confined and under elastic stress and typically released by proximal withdrawal of a confining sheath while the stent is itself prevented from moving proximally with the confining sheath by abutment with a stop on the distal end of a catheter shaft which suffers axial compressive stress while the surrounding sheath is proximally withdrawn.
By contrast, stainless steel stents which are relaxed in a radially compact disposition suffer plastic deformation when expanded into their deployed disposition by inflation of a balloon within the lumen of the stent.
An early example of stainless steel self-expanding stents is Gianturco U.S. Pat. No. 4,580,568 and an early example of the balloon expansible stainless stent is Palmaz EP-A-221 570.
A third category of stent is the memory metal stent, made of a biologically compatible nickel-titanium shape memory alloy with martensitic and austenitic phases. At body temperature, the stent “seeks” to revert to the austenitic phase. Typically it is confined within a surrounding sheath and again released at the stenting site by proximal withdrawal of this sheath.
The present invention offers improvements in systems to deliver those stents which are brought to the stenting site within a confining surrounding sheath.
In the technical field of stenting, there is a desire to reduce the transverse dimensions of the stent delivery system. In this field, the widely used measure of transverse cross-section is the unit of “French”, often abbreviated to “F” which is a one third part of a millimeter. Thus, a 6F (six French) delivery system has a diameter of 2 millimeters.
For any particular stenting operation, one has to select a particular stent and a particular delivery system. There is a large choice in both of these elements. Accordingly, it would be an advantage for manufacturers of stents and their delivery systems to achieve a degree of modularity in the design and construction of stents and their delivery systems. For example, there is a wide range of stents which could be delivered by a six French delivery system and it would therefore be convenient for the manufacturer of a stent delivery system to be able to tailor a basic six French system to fit any particular stent which would be compatible with a six French delivery system. This would reduce costs, to the advantage of patients, while retaining full flexibility for medical practitioners to optimise their choice of stent for any particular patient.
Like many catheter systems, a stent delivery system is often used with a flexible guidewire. The guidewire is preferably made of metal, and is slidably inserted along the desired body passage. The delivery system is then advanced over the thus pre-placed guidewire by “backloading” or inserting the proximal end of the guidewire into a distal guidewire port leading to a guidewire lumen defined by the delivery system.
Many conventional delivery systems define guidewire lumens that extend along the entire length of the outer sheath. These delivery systems are described as “over-the-wire” delivery systems, in that the delivery system is guided to the site of the stenosis over the guidewire, the guidewire thereby exiting the delivery system at the proximal end of the delivery system. “Over-the-wire” delivery systems provide several advantages, including improved trackability, the ability to flush the guidewire lumen while the delivery system is inside the patient's body, and easy removal and exchange of the guidewire while the delivery system remains in a desired position in the patient.
In some circumstances, however, it may be desirable to provide a “rapid exchange” delivery system, which offers the ability more easily to remove and exchange the delivery system while retaining the guidewire in a desired position within the patient. In a rapid-exchange delivery system, the guidewire occupies a lumen located only in the distal portion of the delivery system. The guidewire exits the delivery system through a proximal guidewire port, closer to the distal end of the delivery system than to its proximal end, and extends in parallel along the outside of the proximal portion of the delivery system.
Because a substantial length of the guidewire is outside the delivery system, it may be manually held in place close to the point where it passes the entry point on the body of the patient, as the delivery system is removed. This facilitates handling, removal and exchange of the delivery system for the practitioner for the following reasons.
With a guidewire lumen that is much shorter than the full catheter length a single physician can insert and remove a stent delivery system into and from the patient's body. Whereas over-the-wire delivery systems require a guidewire having a length at least double the length of the outer catheter, the rapid-exchange configuration allows the use of much shorter guidewires which enable a single physician to handle the proximal end of the guidewire at the same time as the catheter at the point of its entry into the body of the patient.
Accordingly, the present invention advantageously provides a stent delivery system having a rapid-exchange configuration for delivering and deploying a self-expanding stent.
Stents themselves cannot be directly seen during their journey to the stenting site, nor can one directly see whether the stent is exactly located as desired within the stenting site. Rather, indirect means have to be used to follow the progress of the stent through the body and make sure that it is correctly located before it is deployed. Thus, a stent delivery system is used during deployment to carry radiopaque contrast or marker fluid to the stenting site so that the target stenosis can be seen through the reduced amount of radiopaque fluid in the bodily lumen at the stenosis. This radiopaque fluid is generally injected through an injection port at the proximal end of the delivery system and through an annular space between an outer sheath of the delivery system and a proximal portion of an inner catheter shaft. The visibility of the stenosis is adversely affected when the lumen, through which radiopaque contrast fluid is injected, is too small to deliver a strong pulse of contrast fluid. As pulses of fluid are used for visualisation, the effectiveness of visualisation depends on the volume flow in each pulse. This in turn depends on the ease of flow of the fluid along the full length of the delivery system, from the point of injection at the proximal end, to the stenosis beyond the distal end of the delivery system.
Thus, delivery systems which offer a large cross-section and unimpeded lumen for contrast fluid will be favoured by radiologists, other things being equal. The visibility can additionally be increased by further reducing the resistance of the system to pulses of contrast fluid. It is therefore an object of the present invention to provide good visualisation with contrast fluid, without sacrifice of other important performance aspects of the delivery system, including pushability and low overall diameter. By increasing “pushability” we mean the capability to be advanced longer distances along narrower and more tortuous bodily lumens.
Furthermore, the delivery system invariably carries at least one radiopaque marker at a known location relative to the length of the stent, so that radiologists can be sure of the location of the ends of the stent, on the basis of their knowledge of the location of the radiopaque marker. Even if the stent is rendered sufficiently radiopaque for it to be seen, it is still useful to have a radiopaque marker on the distal end of the delivery system, to reveal successful separation of the stent from the delivery system.
Thus, in our example of a six French delivery system, to be used for delivering stents of various lengths, there will be a wish to provide radiopaque markers within the delivery system at two spaced-apart locations on the axis of the delivery system, corresponding to the opposite ends of the stent (until the stent is deployed out of the system). One object of the present invention is to offer a degree of modularity in this design aspect.
With delivery systems having a rapid-exchange configuration, just as with over-the-wire systems, the stent delivery system is advanced over the guidewire, itself normally within a guide catheter, in order to bring the distal tip and stent to the stenting site. Depending on the application, different diameter guidewires are specified. Two commonly used guidewire diameters are 0.46 mm/0.018 inches and 0.89 mm/0.035 inches (commonly known as 18 thou or 35 thou guidewires). Thus, a further degree of modularity can be achieved by offering a delivery system which is compatible with a range of guidewire diameters, specifically, both 18 thou and 35 thou guidewires.
It would be an advantage for any new stent delivery system to be able straightforwardly to take the place of those previous delivery systems which individual practitioners have grown to be comfortable using. One such system uses in its proximal portion a metallic rod, which can be either solid or hollow, made of stainless steel.
Good design for stent delivery systems is indicated by manufacturing steps which can be performed with high precision and reliability, yet with acceptable cost levels. This is yet another objective of the present invention.
Finally, for any system which is extremely long in proportion to its diameter, and features at least three co-axial elements, the cylindrical surfaces of these co-axial elements need to be so composed and conformed that friction between them is low enough that the co-axial elements can be moved tolerably easily axially relative to each other. It is yet another object of the present invention to provide systems which enable bringing these friction levels down to advantageously low levels.
Another consideration when a self-expanding stent is released progressively by successive proximal stepwise movements of the outer confining sheath results from the delivery system typically being extremely long in proportion to its cross-sectional dimensions, and constructed predominantly or wholly from synthetic polymeric materials which have substantial elasticity and marked kinetic aspects to their deformation characteristics. In such a case, any particular rate of strain imposed on the proximal end of the outer sheath is likely to be experienced at the distal end of the same sheath in a somewhat different strain rate. For example, a fast squeeze of the trigger of a deployment system at the proximal end of the sheath will likely result in a somewhat slower resulting proximal advancement of the distal end of the same sheath. Furthermore, a pull on the sheath will impose compressive stresses along the length of the inner shaft, likely leading to a proximal movement of the stent which then relaxes back to the original, more distal, position of the stent as the tensile stress in the outer sheath eases back towards zero. In its own delivery systems, present applicant has observed what happens at the distal end of a stent delivery; system during successive squeezes of the trigger of a delivery system which pulls the outer sheath proximally in a series of steps. The appearance at the stent end of the system is as if the system were “breathing” in that it, and the stent, moves axially first proximally, then distally, with each squeeze of the trigger.
This “breathing” phenomenon is of course a complicating factor when it comes to precision of placement of the stent within any particular stenting site. It is yet another object of the present invention to ameliorate this problem.